Thermoelectric materials have been known for many decades now. Early work by F. D. Rosi and others has shown that the pseudobinary system Bi.sub.2 Te.sub.3 -Sb.sub.2 Te.sub.3 and the pseudoternary system Bi.sub.2 Te.sub.3 -Sb.sub.2 -Te.sub.3 Sb.sub.2 Se.sub.3 are useful materials for thermoelectric applications. See W. M. Yin, et al, "Thermoelectric Properties of Bi.sub.2 Te.sub.3 -Sb.sub.2 Te.sub.3 Sb.sub.2 Se.sub.3 Pseudo-Ternary Alloys in the Temperature Range of 77.degree. to 300.degree. K", Journal of Material Science 1 (1966) 52-65. However, the fabrication methods of the prior art have been limited to small areas. Typically these materials have been produced by melting and casting ingots which are then cut into devices or by powder metallurgical techniques. These prior an process are not readily scaled to large area and are limited in their starting materials.
Other processes such as evaporation or chemical vapor deposition (CVD), which are scaled to large area depositions have their own problems. For example, CVD cannot use solid materials but must use gaseous or liquid starting material compounds, which are then broken down by thermal or plasma energy. Typically, the deposited materials in a CVD contain unwanted elements which are incorporated from incompletely reacted starting gases and it is often difficult or impossible to eliminate these contaminants. Evaporation on the other hand can use solid or liquid starting materials, but cannot induce the chemical reactivity and depositing species mobility often needed to achieve the final material properties desired. Therefore, these processes are plagued by an inability to modify or tailor the as deposited composition.
While the method and apparatus of the instant invention is applicable to any material system where it is desirous to deposit large area materials from diverse reactants, one particular type of system is thin-film thermoelectric materials.
The world supply of fossil fuel for the production of energy is being exhausted at an ever increasing rate. This as resulted in a continuing energy and economic crises which impacts not only on the world's economy but on peace and stability. Solutions to the energy crisis include the development of new fuels and the development of more efficient technologies to utilize existing fuels. One method of more efficiently utilizing existing fuels, including energy conservation, power generation, environmental protection, and economic growth, is the thermoelectric generation of electricity.
In the thermoelectric generation of electricity electrical power is generated by heat. It has been estimated that two-thirds of all energy, for example from automobile exhausts, fossil fuel, power plants, and the like, is discharged to the environment without further recovery. This so-called waste heat is paid for and then discharged into the environment without use. Employment of waste heat for the generation of electricity can provide a direct reduction in thermal pollution and an increase in economically efficient utilization of fuels, independent of the original source of the thermal energy.
New, improved, low cost thermoelectric materials and devices will allow for: 1) Reliable, environmentally sound non-CFC cooling systems with no moving parts; 2) low weight power generation devices; 3) low weight cooling/heating devices; 4) waste heat utilization; 5) the ability to provide localized cooling/heating, or power in remote areas; 6) non-dependence on fossil fuels; and 7) battery/solar powered military equipment for personnel comfort devices and hostile environments. Reductions in weight, cost, and improvements in efficiency of the active materials will generate a rapid increase in the utilization of thermoelectric modules for consumer products, military/aerospace, industrial and scientific applications. A very attractive way for obtaining lightweight, low cost thermoelectric modules is to develop a suitable technique for depositing the active materials over large areas on substrates in the form of thick/thin films.
The performance of a thermoelectric device can be expressed in terms of a figure of merit (Z) for the material forming the device. Z is defined by the relationship: EQU Z=S.sup.2 K.sup.-1 .sigma.
where
Z is the Figure-of-Merit PA1 S is the Seebeck coefficient PA1 K is the thermal conductivity, and PA1 .sigma. is the electrical conductivity.
In order for a material to be suitable for thermoelectric power generation, the thermoelectric power coefficient, that is the Seebeck coefficient, S., must be high, the electrical conductivity, sigma, must be high, and the thermal conductivity, K, must be low. For glassy materials K is low.
Therefore, in order for material to be efficient for thermoelectric power conversion, charge carriers must diffuse easily from the hot junction to the cold junction while maintaining a temperature gradient between the two junctions. Thus high electrical conductivity is required along with low thermal conductivity.
Historically, thermoelectric power conversion has not found wide commercial usage. The major reason for this has been that thermoelectric materials which were suitable for commercial applications have been crystalline. Those crystalline materials which are best suited for thermoelectric devices have been difficult to manufacture because of poor mechanical properties and extreme sensitivity of material properties to macroscopic compositional changes. This is because prior art crystalline thermoelectric materials contain a predominance of chalcogenide elements, tellurium and selenium. Tellurium and selenium are natural glass formers. It is because of this tendency of tellurium and selenium to form glasses that the growth, control, and mechanical stability of prior an thermoelectric crystalline materials has been substantially nonreproducible. In addition, the thermoconductivity of glasses are low.
The chalcogenides, such as tellurium, only grow high quality, single crystals with great difficulty. Even when tellurium containing single crystals are gown, the crystalline materials are unstable materials with large defect densities, and compositions far from stoichiometric. For these reasons, controlled doping has proven to be extremely difficult.
Moreover, crystalline solids have been unable to attain large values of electrical conductivity while simultaneously retaining low thermal conductivity.
Conventional polycrystalline thermoelectric materials are (BiSb).sub.2 (SeTe).sub.3 Pb Te, and Si--Ge. The bismuth-antimony tellurides represent a continuous solid system in which the relative amount of bismuth and antimony are from 0 to 100%. Polycrystalline materials also present problems in that the polycrystalline materials have polycrystalline grain boundaries, resulting in relatively low electrical conductivities. Moreover, fabrication of polycrystalline thermoelectric materials into suitable thermoelectric devices have presented difficulties, such as the inability to make lare area, high quality devices devices and the mechanical integrity of the devices during operation.
Improved thermoelectric materials have been developed which are not single phase crystalline materials, but are instead, disordered materials. These materials, more fully disclosed in U.S. Pat. No. 4,447,277 in the names of T. J. Jayadev and On Van Nguyen for "New Multiphase Thermoelectric Alloys and Methods of Making the Same", issued May 8, 1984, incorporated herein by reference. The materials of Jayadev and Nguyen are multiphase materials having both amorphous and multiple crystalline phases. These materials are good thermal insulators, and include grain boundaries of various transitional phases varying in composition from the composition of matrix crystallites to the composition of the various phases in the grain boundary region. The grain boundaries are highly disordered with the transitional phases including phases of high thermal resistivity to provide high resistance to thermal conduction. The materials of Jayadev and Nguyen have grain boundaries defining regions which include conductive phases therein, providing numerous electrical conduction paths through the bulk material for increasing electrical conductivity without substantially affecting thermal conductivity. In essence, the materials have all the advantages of polycrystalline material, with desirably low conductivities and crystalline bulk Seebeck properties. Moreover, the disordered multiphase materials also have high electrical conductivity. Thus the materials of Jayadev and Nguyen have an S.sup.2 (sigma) product for the figure of merit which can be independently maximized with desirably low thermal conductivities for thermoelectric power generation.
The materials of Jayadev and Nguyen are fabricated in a manner which introduces disorder into the material on a macroscopic level. This disorder allows various phases, including conductive phases, to be introduced into the materials.
U.S. Pat. No. 4,588,520 by T. J. Jayadev, On Van Nguyen, Jaime M. Reyes, H. Davis, and M. W. Putty, (hereinafter "Jayadev et al"), incorporated herein by reference, describes compacted an/or compressed powder materials useful for thermoelectric applications. The powdered materials have compositional disorder, translational disorder, configurational disorder, and other disorders introduced therein. The powder materials are multiphase alloy materials having a first phase, including matrix crystallites bounded by disordered grain boundaries at various phases including transitional phases. Between the grain boundaries are macroscopic grain boundary regions which also include various phases, including electrically conductive phases and crystalline inclusions. The grain boundary regions are rich in electrically conducting modifying phases which provide high electrical conductivities. The other phases in the grain boundary regions and the grain boundaries provide low thermal conductivities.
The compacted materials further include additional bulk disorder between the interfaces of the compacted powder particles which further reduce thermal conductivity. The materials comprise a body formed from compacted powder material. The compacted powder material includes bismuth, tellurium, and a t least one highly electrically conductive phase.
The materials described in Jayadev et al are made by forming a mixture containing the constituent elements of a first compound including at least bismuth and tellurium and constituent elements of a second compound capable of forming at least one highly electrically conductive phase, and thereafter compressing at least a portion of the particulate mixture to form a compacted body of the material. The first and second compounds may be separately prepared from the respective constituent elements, and then the first and second compounds combined and heated to form a melt, with the melt cooled to form a solid material form which is crushed to form the particulate materials.
Alternately, a melt may be formed from the second compound and the constituent elements of the first compound and then cooled, for example by planar flow casting, to a solid material form and crushed to form the particulate mixture. According to further alternative, the first and second compounds, that is the bismuth and tellurium compound, and the compound capable of forming at least one highly electrically conductive phase, may be separately prepared from their respective constituent elements and separately crushed into particulate form to form the particulate mixture.
The first compound includes bismuth, antimony, and tellurium for making a p-type material and bismuth, tellurium, and selenium for making an n-type material. The second compound, to be combined with either of the first compounds, that is, with either the p-type material or the n-type material, includes silver, antimony, and tellurium.
Individual thermoelectric elements of the n-type drive negative carders from the hot side of the device to the cold side of the device, while individual thermoelectric elements of the p-type conductivity drive positive carders from the hot side of the device to the cold side of the device. Operative thermoelectric devices are characterized by a plurality of thermoelectric elements, thermally in parallel and electrically in series. N-type elements an p-type elements of the thermoelectric device are assembled so that they are thermally in parallel and electrically in series with one another. Each pair of elements contain one n-type thermoelectric element and one p-type thermoelectric element electrically connected at one end by an electrical connecting strap. Each strap connects the end of an n-type element of each pair of electrically connected thermoelectric elements to the p-type element of the next adjacent pair of electrically connected thermoelectric elements. Thus, all of the individual n-type and p-type thermoelectric elements of a thermoelectric device are connected electrically in series and thermally in parallel. In order to achieve maximum power output for a thermoelectric device, the electrical resistance of both the individual strap and of the thermoelectric device to strap contact must be minimized.
While the thermoelectric materials have been known and used for many years, to date there has been little investigation into production of apparatus to deposit large area, uniform thermoelectric materials. The present invention discloses a microwave enhanced plasma deposition apparatus and method for fabricating unique thin/thick-film compositions at relatively low cost from diverse starting materials which include solid or liquid sources. The apparatus combines both physical vapor deposition (PVD) with chemical vapor deposition (CVD) and can accommodate simultaneous use of solid/liquid starting materials with gaseous starting materials. The process allows for control of the morphology, chemistry and composition of the as-deposited materials to obtain the desired material properties.